The distance from a facility beyond which, for screening purposes, potential origins of a particular type of external event can be ignored. The definition in NS-G-3.1 had 'sources' instead of 'origins'
International Atomic Energy Agency IAEA, Department of Nuclear Safety and Security, IAEA Safety Glossary (Terminology Used In Nuclear Safety and Radiation Protection), IAEA, Vienna. Austria 2007. ISBN 92-0-100707-8, http://www-pub.iaea.org/MTCD/publications/PDF/Pub1 290_web.pdf, (Nucl Sfty and Rad Prot - Brit) {459}